Authentication system and method are provided. The authentication system includes: a server configured to provide at least two security levels and configured to transmit one of at least two security modules corresponding to the security level of a user terminal, via communications network, to the user terminal based, at least in part, upon an environment of the user terminal; and an authentication server communicatively linked with the server and configured to perform a user authentication in response to a user authentication request from the user terminal. Accordingly, various hackings can be prevented and the user authentication can be accomplished with user's convenience and security.